1. Field of the Invention
The present invention relates generally to the biomedical arts, and, in particular, to an improved multi-channel device which finds particular application in introducing a string of artificially generated antidromic pulses on the nerve trunk for collision blocking orthodromic pulses moving in the opposite direction along the nerve trunk and will be described with particular reference thereto. It is to be appreciated, however, that the invention may have broader applications and may apply electrical signals on nerve trunks for other purposes.
2. Description of the Related Art
Heretofore, various techniques have been used to block nerve pulses passing along a nerve trunk. A common blocking technique was the application of DC currents on the nerve trunk. However, it has been found that the application of DC currents can be expected to cause nerve damage.
To eliminate the DC current induced nerve damage, others have suggested using an oscillating current such that the induced electrical current flowed alternately in both directions along the nerve trunk. It has been found that the application of high frequency stimulation blocks the passage of nerve signals therethrough. However, it appears that high frequency stimulation may, in effect, be overdriving neuromuscular junctions and depleting the neurotransmitter at the terminal end. That is, rather than blocking the passage of nerve stimuli on the nerve fiber or axon, the high frequency stimulation techniques may be overworking the nerve terminal to the point of exhaustion causing a failure of proper functioning.
Yet another blocking technique utilized a three electrode cuff which included a dielectric sleeve having a passage through which the nerve trunk passes. Three annular electrodes were arranged within the sleeve. A cathode was positioned near the center of the passage and a pair of anodes were positioned to either side. A signal generator was connected with the electrodes to apply an electrical pulse train that induced antidromic pulses on the nerve trunk. Each pulse of the pulse train included a rapid rise to a preselected amplitude, a 100 to 3000 microsecond plateau, and an exponential decay back to zero. This pulse train induced artificially generated antidromic pulses on the nerve trunk which traveled unidirectionally in the opposite direction to the normal pulse flow. The artificially generated antidromic pulses collided with and blocked further propagation of natural orthodromic pulses moving in the other direction on the nerve trunk. However, the application of a series of pulses of common polarity, again has been found to cause damage to neural tissues.
To eliminate this nerve damage, others have suggested applying a low amplitude, relatively long duration rectangular wave pulse of opposite polarity between each pulse of the above-described pulse train. The opposite polarity of the rectangular wave pulse balanced the net charge flow caused by the primary pulse. However, it has been found that at an upper limiting frequency, the sudden polarity change still tends to depolarize the nerve cell and cause transmission in the wrong direction. This tendency to generate artificial orthodromic pulses, of course, was undesirable. For example, if the antidromic blocking pulses were utilized to block stray excitation pulses moving toward a paralyzed patient's spastically contracted sphincter muscle over which control had been lost, the stray orthodromic pulses would cause undesired activation of the muscles of micturition.
U.S. Pat. No. 4,608,985 provides a system for selectively blocking orthodromic action potentials passing along the nerve trunk. The system includes an electrode cuff including a cathode disposed around the nerve trunk and a dielectric shield disposed encircling the electrode and the nerve trunk to both sides of the electrode. An anode is electrically associated with body tissue such that electrical current flows from the anode through the body tissue and nerve trunk to the cathode. A signal generator is operatively connected with the cathode and anode for cyclically generating electrical pulses. Each pulse cycle includes a first polarity pulse which rises abruptly to a first preselected amplitude, retains the amplitude for a preselected duration, and decays smoothly from the amplitude. Each cycle further includes an opposite polarity phase whose leading edge is a smooth continuation of the first polarity pulse decaying trailing edge. The opposite polarity pulse rises smoothly to a magnitude whose absolute value is less than the first polarity pulse magnitude and which is too low to trigger action potentials. The opposite polarity pulse is substantially longer than the first polarity pulse such that the charge flow during the first and opposite polarity pulse is opposite but generally equal.
Although the system described in the aforementioned patent has been found to be adequate for sacral root stimulation, its original design was intended for percutaneous type of stimulation where the connection between the stimulator and the electrodes is made outside of the body, with the electrode leads penetrating the skin from outside of the body. Such a percutaneous system is prone to infections, lead breakage, and difficulty in maintaining a reliable connection between the leads and the stimulator due to patient movement. Thus, an implantable system is often desirable for many applications in the field of functional electrical stimulation, especially those which require long term stimulation.
The major obstacle which makes it difficult to transform this system from a percutaneous to an implantable device lies in the fact that the circuitry for providing stimulation to each sacral root must be kept isolated. This isolation can be maintained by the use of separate power supplies (batteries) for each channel. Thus, in a device which stimulates six sacral roots, the system employs six separate batteries to provide six isolated output channels. A direct translation of this percutaneous device means that an implantable device must also use six batteries in order to achieve isolation. The use of six batteries would increase the size and weight of an implantable device such that the device may be impractical.
The importance of isolated output channels in a device using a single power supply stems from the fact that stimulation via non-isolated outputs has shown a decreased ability to selectively stimulate the proper sacral roots when compared to isolated outputs. This is due to the leakage currents which exist in non-isolated systems that create unwanted current pathways between the nerve fibers, potentially causing unwanted nerve excitation and damage to the electrodes and also the tissue.
The present invention provides a new and improved device which can overcome the above referenced problems.